Passive keyless entry system

ABSTRACT

A passive keyless entry system that is specifically adapted for use with automotive vehicles is designed to automatically unlock the vehicle as the operator approaches the vehicle. The system is further designed to automatically lock the vehicle as the operator, carrying the beacon, moves away from the vehicle. The system includes a portable beacon that is carried by the operator, a receiver/controller located in the vehicle, and an antenna connected to the receiver/controller for receiving the encoded transmission from the beacon. The beacon includes a motion sensor to conserve battery life when the beacon is stationary. Transmission between the beacon and the receiver/controller is characterized by a magnetically coupled radio frequency signal embodying differential phase encoded data with error correction coding of the data to enhance noise immunity and signal discrimination.

This is a continuation of U.S. patent application Ser. No. 08/164,055,filed Dec. 8, 1993 now abandoned, which is a continuation of U.S. patentapplication Ser. No. 07/513,900, filed Apr. 24, 1990, now U.S. Pat. No.5,319,364, which is a division of U.S. patent application Ser. No.199,476, filed May 27, 1988, now U.S. Pat. No. 4,942,393.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to electronic keyless entry systems and inparticular to a passive keyless entry system that is particularlyadapted for use with automotive vehicles.

Automotive keyless entry systems, when first introduced, typicallyincluded a numerical keypad located on the exterior door panel of thevehicle. The operator entered a unique multiple digit code on the keypadto automatically unlock or lock the vehicle. Recently, moresophisticated keyless entry systems for vehicles have been proposedwhich use a portable remote transmitter that is carried by the operatorand a receiver located in the vehicle that is adapted to unlock thevehicle in response to the detection of a coded radio frequency signalor a coded optical signal received from the transmitter. Such systemsrequire that the operator actuate a button or switch on the transmitterto initiate the transmission, similar to the operation of a conventionalautomatic garage door opener, in order to conserve battery life andprevent inadvertent actuation.

While more convenient to operate than the keypad-type keyless entrysystems, the latter transmitter/receiver-type systems nonethelessrequire that the operator physically locate the transmitter and actuatea button to unlock the vehicle. Hence, the convenience provided by sucha system versus a conventional key and lock arrangement is notsubstantially improved.

In addition, while other types of "keyless" entry systems are known andpresently used in other applications, these systems for various reasonsare generally not suitable for automotive use. This would includesystems employing magnetic card readers, interrogation/transponder-typesystems, and conventional automatic garage door openers. A magneticcard-type system is probably adaptable to automotive use, but provideslittle added benefit to justify the expense.Interrogator/transponder-type systems, though adaptable to operate in atotally passive manner, are more complex and therefore more expensive,and present a power consumption problem as the interrogator thereforemust be on and interrogating at all times. In addition, the transponderin such systems must be capable of receiving as well as transmittingdata, thus adding to cost. Lastly, automatic garage door systems, whileappearing to employ similar technology have substantially differentoperating requirements which result in significantly different circuitdesigns. In particular, an automotive keyless entry system must possessa level of noise immunity and signal discrimination that issubstantially greater than that required for an automatic garage dooropener. This is due to several factors including the many differentenvironments in which vehicles may be located, the greater number ofvehicles that may be equipped with comparable systems, and the fact thatlarge numbers of vehicles are frequently located within close proximityto one another, such as in parking lots. In addition, automatic garagedoor systems require that the operator actuate a transmitter, andtherefore are not totally passive. Also, garage door opener-type systemshave a substantial range and therefore can potentially activate afunction, such as unlocking the trunk of the vehicle, when the operatoris some distance away from the vehicle and unaware the trunk has beenopened.

Accordingly, it is the primary object of the present invention toprovide a totally passive keyless entry system that is especiallyadapted for use with automotive vehicles.

It is also an object of the present invention to provide an automotivekeyless entry system that is adapted to automatically unlock the vehicleas the operator approaches the vehicle.

In addition, it is an object of the present invention to provide apassive keyless entry system having a beacon/transmitter that isextremely small in size and includes a motion sensing switch thatautomatically activates the transmitter in the beacon whenever movementof the beacon is sensed.

It is a further object of the present invention to provide a passivekeyless entry system that employs electronic circuitry which permits thesystem to function in the micropower range while in its quiescent state.

Additionally, it is an object of the present invention to provide apassive keyless entry system having an acceptable operating rangebetween the beacon and receiver while providing a projected beaconbattery life in excess of one year.

Further, it is an object of the present invention to provide a passivekeyless entry system that employs signal transmission and codingtechniques which provide a high level of noise immunity and signaldiscrimination.

It is also an object of the present invention to provide a passivekeyless entry system that is reliable and yet is relatively inexpensiveto manufacture.

In general, the passive keyless entry system according to the presentinvention comprises three basic components: a transmitter or beacon, areceiver/controller, and a receiving antenna. The beacon, which is smallenough to be attached to a keychain, is carried by the operator andincorporates a motion sensor that is used to energize the transmitterportion of the beacon. The transmitter in the beacon is adapted to emita coded radio frequency signal that contains both identification andfunction information and an error correction code. The beacon isdesigned to continue to transmit repeatedly its coded signal until nomotion is detected for a predetermined period of time. Thus, the motionsensor serves to promote beacon battery life and enables the presentsystem to function in a totally passive manner.

The receiving antenna comprises a simple coil of wire, wound to besensitive at the transmitted frequency of the beacon. The antenna islocated at a position on the vehicle to optimize the performance of thesystem. It has been found desirable to adapt the system so that thereceiver/controller is responsive to the signal from the beacon when thebeacon comes within 3-6 feet of the vehicle.

The receiver/controller is mounted inside the vehicle and is adapted tooperate on the vehicle's 12 volt negative ground battery. Upon receiptof a radio frequency signal from the antenna, the receiver/controller isadapted to process the coded radio frequency signal and evaluate theserial data contained therein. If the signal is determined to be valid,the receiver/controller automatically unlocks the driver's-side vehicledoor.

Optionally, the beacon may be provided with function switches which,when depressed by the operator, change the function code contained inthe radio frequency signal, thus directing the receiver/controller toopen the vehicle trunk and/or perform other functions. In addition, thereceiver/controller in the present system is adapted to automaticallylock the vehicle doors when the operator leaves the vehicle and carriesthe beacon out of range of the receiver/controller, assuming the properstatus of the ignition and doorjamb switches.

Additional objects and advantages of the present invention will becomeapparent upon reading the following description of the preferredembodiment of the present invention which makes reference to thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical view illustrating the preferred applicationof the present invention;

FIG. 2 is a partial cutaway view of the receiving antenna used in thepresent invention;

FIG. 3 is a timing diagram illustrating the Miller encoding techniqueused in the preferred embodiment;

FIG. 4 is a perspective view of the beacon/transmitter of the presentinvention;

FIG. 5 is a circuit diagram of the beacon/transmitter shown in FIG. 4;

FIG. 6 is a block diagram of the beacon/transmitter shown in FIG. 5;

FIG. 7 is a timing diagram illustrating the various signals produced bythe timing controller 54 shown in FIG. 6;

FIG. 8 is a block diagram of a linear N_(p) stage shift register used togenerate the error correction code polynomial used in the preferredembodiment;

FIG. 9 is a diagrammatical view of the steps used to generate thetransmitted beacon code;

FIG. 10 is a detailed circuit diagram of the antenna driver circuit 66of FIG. 6;

FIG. 11 is a combined timing and circuit diagram illustrating the mannerin which the bipolar driver signal is generated;

FIG. 12 is a signal diagram illustrating the waveform of the beaconantenna;

FIGS. 13a-13c are circuit diagrams of the receiver/controller accordingto the present invention;

FIG. 14 is a block diagram of the bipolar front end and digital datadetect custom integrated circuits used in the receiver/controller;

FIG. 15 is a detailed block diagram of the clock generator circuit shownin FIG. 14;

FIG. 16 is a detailed block diagram of the carrier synchronizer circuitshown in FIG. 14;

FIG. 17 is a detailed block diagram of the lock detector circuit shownin FIG. 14; and

FIG. 18 is a detailed block diagram of the data extraction circuit shownin FIG. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a diagrammatic view illustrating the application ofthe present invention as an entry system for an automobile 10 is shown.As depicted in the drawing, the beacon (concealed) is adapted to becarried by the driver 12 and is responsive to the motion created as thedriver walks to energize the transmitter portion of the beacon. Whileenergized, the transmitter portion of the beacon continuously transmitsa coded radio frequency signal which includes identification andfunction data, and an error correction code. When the driver comeswithin range of the vehicle-approximately three feet in the preferredembodiment-the receiving antenna (also concealed) located on the vehiclemagnetically couples the transmitted beacon energy into thereceiver/controller. The antenna 14 (FIG. 2) in the preferred embodimentis located in the B-pillar 16 of the vehicle as this location has beenexperimentally determined to produce the most favorable results.However, other locations on the vehicle have been found to beacceptable. Moreover, in view of the magnetic coupling between thebeacon and receiver, it is possible to control the field of the antennaby appropriate placement, orientation, and configuration of the antenna,and in this manner provide coverage in selected areas, such as in thevicinity of the trunk lid or the driver-side door handle.

The receiver/controller, which is located in the vehicle, is adapted toprocess the coded radio frequency signal received from the antenna andperform the function requested if the identification code is determinedto be "valid", where a "valid" code corresponds to the code prestored inthe receiver/controller. Optionally, the beacon may be provided with oneor more input switches that can be used to transmit up to sixteendifferent function codes to control various functions in addition tounlocking the driver's side door, such as unlocking the trunk, turningon the interior lights, etc. Preferably, however, when none of thefunction switches are depressed, the receiver/controller is adapted tointerpret the default function code as a command for unlocking thedriver's side door of the vehicle. Alternatively, the "default" functionof the system can be selected to unlock all doors of the vehicle, or anyother function desired by the automobile manufacturer. In addition, thesystem is adapted to respond to the condition where the driver leavesthe vehicle by automatically locking all doors of the vehicle apredetermined period of time after the beacon signal becomes too weak toreceive.

Turning to FIG. 2, a partial cutaway view of the receiving antenna 14used in the preferred embodiment is shown. The antenna 14 merelycomprises a single wire, such as 34-gauge magnet wire, that is wrappedas shown at 18 the number of turns required to provide the desiredinductance. Preferably, the magnet wire is wrapped around a stable formto maintain the shape of the antenna rigid so that the inductance of theantenna does not vary. A plastic shield 20 may be placed over the coilof wire 18 for protection. Although the drawing shows the antenna ashaving a circular shape, other configurations can be used (e.g.,rectangular) to accommodate space constraints at the desired mountinglocation. In addition, it may be desirable to add a second grounded wireto the coil for electrical shielding purposes to improve the performanceof the antenna. However, this also adds to the cost of the antenna.

Communication Channel

In order to conserve the battery power of the beacon and the vehiclebattery, the transmission frequency in the preferred embodiment isselected to be a relatively low 98.3 KHz. The frequency of thetransmission signal is therefore substantially above most of the lowfrequency noise that emanates from an automobile and yet the fifthharmonic of the transmission frequency is still below the AM radio band.Accordingly, interference with AM radio frequency signals is avoided.The beacon antenna comprises a coil of wire as described that is tunedto the selected carrier frequency. The antenna creates a magnetic fieldwhich couples to the coil of wire used for the receiving antenna. Thechange in flux caused by the receipt of a signal at the carrierfrequency generates a voltage across the receiving antenna terminals.When the beacon is placed in the center of the receiving antenna, theresulting voltage can be as high as 2 volts peak-to-peak. However, asthe beacon moves a few feet away from the center of the receivingantenna, the voltage may drop to less than 15 microvolts peak-to-peak.This wide variation in signal level comprises one of the majorconstraints in the selection of appropriate communication channelarchitecture.

Although many different types of carrier modulation schemes may beemployed, including frequency shift keying, amplitude shift keying,etc., phase shift keying ("PSK") has been selected for the preferredembodiment. In PSK encoding, the frequency stays the same, but the phaseis shifted exactly 180 degrees to differentiate a logical "1" from alogical "0". This scheme uses a minimum bandwidth, and is very simple toimplement in the transmitter. In particular, PSK encoding may beaccomplished using an exclusive-OR gate with the carrier frequencysupplied to one input and the data signal provided to the other. Anadditional benefit of PSK encoding is energy efficiency. Specifically,PSK encoding uses all of the transmitted energy for information with noenergy being separately applied to generate a carrier. Moreover, frontend signal processing of PSK encoding can be accomplished at thereceiver using a limiter instead of an automatic gain control circuit.Hence, since PSK encoding is defined in terms of phase, the receivedsignal will not suffer when passed through a limiter. The primarydisadvantage of PSK encoding, on the other hand, is the need toregenerate the carrier at the receiver and thus properly synchronize thereceiver to the coded transmitted signal.

Error Correction Code

While the use of many different coding schemes is possible, thepreferred embodiment of the present keyless entry system uses a codingscheme that employs an error correction code to improve systemperformance. Specifically, the use of an error correction code allowsthe system to properly receive a coded transmission which may have beenpartially altered by noise. This improves reception performance and theresponse time of the system. In addition, the use of error correctioncodes results in a reduction of false detections. A false detectionoccurs any time the receiver/controller indicates the detection of avalid beacon key code when the valid code was not transmitted. This canoccur due to random noise, or the presence of another beacon with asimilar code that is incorrectly decoded into a valid code.

The coding scheme used in the preferred embodiment comprises a 48-bitcode with two repetitions. The first four bits in the coded transmissioncomprise preselected synchronization bits which produce eighttransitions that are illegal under the Miller code. The four SYNC bitsare followed by a 4-bit FUNCTION code and a 20-bit IDENTIFICATION code.A 24-bit error correction code ("ECC") is then added to the 24 bits ofFUNCTION and IDENTIFICATION code. The 4-bit FUNCTION code, as previouslynoted, provides up to sixteen different FUNCTION codes to selectivelycontrol the activation of additional functions as desired, such as trunkunlock, unlock all doors, turn on interior lights, etc. In addition, itwill be appreciated that the 20-bit IDENTIFICATION code provides inexcess of one million different ID codes, thereby significantly reducingthe probability that two beacons with the same ID code will be presentin the same vicinity at the same time.

In that the SYNC bit pattern is the only mechanism that thereceiver/controller has to synchronize onto the data for the purposes ofMiller decoding and extraction of the data bits, it is possible fornoise to generate a pattern which will cause the receiver/controller tobegin to read a code word at the wrong place. Moreover, it is the natureof cyclic codes that any shifted version of a legal code word is itselfa legal code word. The improperly synchronized code word could,therefore, become a valid code word for another vehicle and thus causean improper unlocking of a door or activation of another function. Toovercome this problem, the coding scheme in the preferred embodimentemploys an anti-slip pattern that is superimposed onto the 48-bit codeword making an improperly synchronized code word invalid on othervehicles. Specifically, the 48-bit FUNCTION, IDENTIFICATION, and ECCcode word is exclusive-OR'ed with a predetermined 48-bit anti-slippattern. The resulting output from the exclusive-OR gate is thenprovided to a Miller encoder which produces the final encodedtransmitted word.

The reason for employing the Miller encoding technique is as follows.One of the characteristics of PSK encoding is the ambiguity at thereceiver. This occurs because of the exact 180 degree phase the shift.Since the signal received for a digital "1" is essentially the negativeof the signal transmitted for a digital "0", it is impossible todistinguish one from the other without some prior knowledge. Thisproblem is effectively eliminated by using an edge sensitive dataencoding scheme such as Miller encoding which has been selected for thepresent keyless entry system. In Miller encoding, when a logical "1" istransmitted, a transition always occurs in the middle of the bit cell.When a logical "0" is transmitted, a transition occurs at the beginningof the bit cell only if the previously transmitted bit was also alogical "0". Otherwise, no transition occurs. Miller encoding isutilized in the preferred embodiment because it is relatively simple toimplement at both the transmitter and receiver/controller.

Miller encoding is defined by the following table:

    ______________________________________    D.sub.n-1            D.sub.n      TD.sub.2n                                TD.sub.2n+1    ______________________________________    0       0            1      0    0       1            0      1    1       0            0      0    1       1            0      1    ______________________________________

where

n is an integer 0. . last n.

D_(n-1) is the previously transmitted data bit. When n=0, D_(n-1) isdefined to be 1.

D_(n) is the current data bit.

TD_(2n) is the first of two transition bits for each data bit.

where

1 indicates a change in the state of the encoded data signal, and

0 indicates no change.

TD_(2n+1) is the second of two transition bits for each data bit.

where

1 indicates a change in the state of the encoded data signal, and

0 indicates no change.

The Miller encoding table is illustrated for convenient reference inFIG. 3.

As previously noted, it is necessary for the receiver to identify thebeginning of the encoded transmission word and for this purpose a 4-bitSYNC pattern is added at the beginning of the code word which presentsan illegal pattern for Miller encoding. Because of this illegal natureof the SYNC pattern, it can always be differentiated by the receiverfrom the code word. The SYNC pattern used in the preferred embodiment isdefined by the transition string: 01000001, where "1" indicates a changein the state of the encoded data signal and "0" indicates no change.

In addition, it will be recognized that the receiver/controller mustseparate the signal coming through the data channel into individualbits. Since the bits are separated in time, this requires a clock. Thepreferred solution to this problem is a self-clocking code. Aself-clocking code is characterized by a reasonable number oftransitions that are well-defined with respect to the data bits. It willbe appreciated, therefore, that since Miller encoding ensures thepresence in the transmitted encoded word of a reasonable number oftransitions, the use of Miller encoding also satisfies the requirementfor a self-clocking code.

Beacon/Transmitter

Turning now to FIG. 4, a perspective view of the beacon/transmitter 24according to the present invention is shown. In the preferredembodiment, the beacon is packaged in a relatively small, generallyrectangular housing 26 approximately 2 inches by 1.5 inches by 0.5inches in size and having an opening 28 for conveniently attaching thebeacon to a keychain. The beacon 24 illustrated in FIG. 4 includes twomanually actuable switches 30 and 32 that are operative to unlock thetrunk of the vehicle and unlock the passenger doors. As previouslyindicated, the beacon may alternatively be provided with additionalfunction switches to control the selective activation of additionalfunctions, such as turning on the interior lights of the vehicle. Asalso previously noted, in the default mode where none of the functionswitches are depressed, the beacon is adapted to transmit the "unlockdriver's-side door" function code.

Referring now to FIG. 5, a circuit diagram of the beacon/transmitter 24according to the present invention is shown. In the preferredembodiment, the beacon circuit is comprised of a single customintegrated circuit 40, a battery 42, a coil of wire acting as atransmitting antenna L1, a motion detector 44, a quartz crystal 46, andthe two function switches 30 and 32, all mounted to a printed circuitboard. The battery 42 utilized in the preferred embodiment is aconventional 3 volt lithium manganese dioxide watch battery. Similarly,the quartz crystal 46 used in the preferred embodiment comprises aconventional 32.768 KHz watch crystal that is used to control theoperating frequency of the beacon. In particular, the energy containedin the third harmonic of the crystal frequency signal is used in theantenna driver. The beacon antenna L1 consists of a coil of wire and atuning capacitor C1, the values of which are tuned to the thirdharmonic, 98.304 KHz. The motion detector 44 is a device that isresponsive to very slight movements and is adapted to continuously cycleon and off when movement is sensed. In addition, the preferred motiondetector is equally sensitive to motion in any orientation of thebeacon. A motion detector suitable for use in the present application isdisclosed in U.S. Pat. No. 4,942,393, assigned to the assignee of thepresent application.

A block diagram of the beacon/transmitter 24 according to the presentinvention is illustrated in FIG. 6. The signal from the motion sensingswitch 44 is provided to a motion detector and oscillator enable circuit50. This interface circuit 50 is always active and is adapted to monitorthe state of the motion sensing switch 44 and produce a timed oscillatorenable signal at its output 51 whenever a change in the state of themotion sensing switch 44 is detected. In the preferred embodiment, thetimed oscillator enable signal comprises a 26-second pulse. If motioncontinues to be detected, successive enable signals will be produced sothat the oscillator enable signal on line 51 will terminate 26 secondsafter all motion of the beacon has ceased. To conserve battery life, themotion detector and oscillator enable circuit 50 is the only circuitthat remains active at all times.

In response to the oscillator enable signal on line 51, the oscillatorcircuit 52 is activated to produce the 32.768 KHz crystal clock signal("XTAL") at its output. The 32.768 KHz crystal clock signal is providedto a timing controller circuit 54 that is adapted to produce the varioustiming signals used for generating the code word, initializing thevarious circuits, and sequence control. The various timing signalsproduced at the output of the timing controller 54 are illustrated inthe timing diagram shown in FIG. 7. In particular, the XTAL divide-by 16and XTAL divide-by 32 clock signals are produced at the clock 0-2!output lines; the 0-51 State Count corresponding to the 48-bit code wordplus the four SYNC bits, is produced at the State Count 0-5! outputlines; and the Parity, Miller, FUNCTION, and IDENTIFICATION signals areproduced at the Control 0-3! output lines. The State 0-1! output line isprovided to the encoded data generator 64 and supplies a sync pulse orend-of-message pulse at the end of each complete cycle.

A debounce circuit 56 is shown connected to four function switchesalthough, as previously noted, the preferred embodiment herein utilizesonly two function switches 30 and 32. The four outputs from the debouncecircuit 56 are connected to the message generator circuit 58 whichcontrols the content of the IDENTIFICATION and FUNCTION informationproduced at the MESSAGE output. The status of the four function inputsdetermines the content of the 4-bit FUNCTION code. The 20-bitIDENTIFICATION code, on the other hand, is set at the manufacturingstage by connecting the 20 input lines (ID0-ID19) to the messagegenerator 58 to prewired fused-link jumpers, or to a preprogrammedEEPROM memory. The MESSAGE output from the message generator 58comprises a serial output signal that is provided to the errorcorrection code (ECC) encoder 60. The ECC encoder 60 is adapted togenerate a 24-bit error correction code based upon the content of theMESSAGE code received from the message generator 58.

The parity bits, or error correction code, are generated using a linearN_(p) stage shift register connected as illustrated in FIG. 8,

where:

g_(i) is the ith coefficient of the generator polynomial

g(X)=1+g₁ X+g₂ X² + . . . +g_(n-k-1) X^(n-k-1) +g_(n-k) X^(n-k),

n is the total number of bits in the code word, in this case 48,

k is the number of message bits in the code word, in this case 24,

b_(i) is the value stored in the ith register,

+ indicates an exclusive OR-gate,

AND indicates an AND-gate,

switch is a multiplexer, and

"Message" is the bit pattern consisting of the FUNCTION code followed bythe ID code.

The specific generator polynomial used in the preferred embodiment is asfollows:

g(X)=1+X=X² +X⁴ +X⁵ +X⁶ +X⁸ +X⁹ +X¹⁰ +X¹³ +X¹⁶ +X¹⁷ +X¹⁹ +X²⁰ +X²² +X²³+X²⁴.

The anti-slip pattern generator 62 generates a fixed 48-bit pattern atthe same clock rate as the serial output from the ECC encoder 60. Aspreviously noted, the anti-slip pattern is superimposed onto the codeword to prohibit the receiver from detecting a valid code from anotherbeacon with a similar code that is shifted in position relative to thevalid beacon code. This serves to reduce the probability of falsedetection from other beacons. The anti-slip pattern used in thepreferred embodiment which begins in bit position 0 is as follows:

    ______________________________________            1100 1100 1011 0101 1111 0000            0000 1111 1010 1101 0011 0011.    ______________________________________

The serial outputs from the anti-slip pattern generator 62 and the ECCencoder 60 are provided to the encoded data generator 64. The encodeddata generator 64 is adapted to exclusively-OR the 48-bit anti-slippattern with the 24-bit MESSAGE and 24-bit ECC code word and then Millerencode the data. In addition, the encoded data generator 64 also addsthe 4-bit SYNC code to the beginning of the Miller encoded data word.The resulting encoded data word is serially provided to an antenna drivecircuit 66 which is connected to the tuned antenna circuit 68.

To summarize, the various steps for generating the transmitted beaconcode are diagrammatically illustrated in FIG. 9. Initially, the 4-bitFUNCTION code is determined in accordance with the status of the variousFUNCTION switches. The 20-bit IDENTIFICATION code which is stored in thebeacon chip, is combined with the 4-bit FUNCTION code to make a MESSAGE.Based upon the content of the MESSAGE, 24 parity bits are generatedusing the ECC polynomial. The parity bits are then appended to theMESSAGE resulting in a 48-bit code word. Each of the 48 bits in the codeword is then exclusive-OR'ed with each bit of a predetermined 48-bitanti-slip pattern. The resulting 48-bit code word is then Miller encodedand a 4-bit SYNC pattern, illegal under the Miller code, is added to thebeginning of the Miller encoded word.

Turning now to FIG. 10, a detailed circuit diagram of the antenna drivecircuit utilized in the preferred embodiment is shown. The antenna drivecircuit is designed to meet the following objectives: (1) high outputefficiency; (2) average battery drain of less than 40 microamps overnormal operating conditions; (3) frequency tripling to allow the use ofan inexpensive 32.768 KHz crystal oscillator; (4) a reasonably cleansignal spectrum; (5) freedom from latch-up during normal operation; and(6) a small number of external components. The preferred embodiment ofthe present beacon/transmitter utilizes a bipolar antenna drive circuit66 which serves to inject as much energy as possible into the antenna toimprove the efficiency of the beacon. The bipolar drive circuit 66produces narrow drive pulses that are provided to the antenna circuit 68every 1.5 cycles, alternately driving the antenna circuit 68 toward thebattery voltage and toward ground. With this approach, an outputcapacitor C2 is required, and the signal decays for only 1.5 cyclesbetween pulse injection. An additional benefit of the bipolar drivecircuit is that the antenna oscillates at a peak-to-peak voltage equalto the battery voltage.

To generate the bipolar drive signal, the XTAL clock signal is providedthrough a time delay circuit 70 to produce a DELAYED XTAL clock signal.The XTAL and DELAYED XTAL clock signals are then provided to the inputsof an AND-gate equivalent 72 and a NOR-gate 74 to produce a first narrowpulse train on line 76 and a second narrow pulse train on line 78shifted by 180 degrees relative to the signal on line 76. The waveformillustrated above the narrow pulse train waveforms represents thephase-shifted difference between the XTAL and DELAYED XTAL clocksignals. The Encoded Data signal on line 80 is clocked through a Dflip-flop 82 by the XTAL clock signal to provide non-inverted andinverted Encoded Data signal on output lines 84 and 86, respectively.

The non-inverted and inverted Encoded Data signals on lines 84 and 86and the first and second narrow pulse train signals on line 76 and 78are provided to a logic gate network 90 having two output lines 92 and94. The logic gate network 90 functions in the following manner. When aclock pulse is present on line 76, the presence of a logic "1" in theEncoded Data word will result in a HI signal pulse being produced onoutput line 94 and the presence of a logic "0" in the Encoded Data wordwill result in a LO signal pulse being produced on output line 92.Conversely, when a clock pulse is present on line 78, the presence of alogic "1" in the Encoded Data word will result in a HI signal pulsebeing produced on output line 92 and the presence of a logic "0" in theEncoded Data word will result in a LO signal pulse being produced onoutput line 94. The output signal on line 92 is inverted by inverter 96and the two resulting parallel data signals are then provided to thegates of two N-type and P-type FET transistors Q1 and Q2, as illustratedin FIG. 11. Note that the period of the two resulting signals providedto the FETS Q1 and Q2 is equal to 1/32.768 KHz or 30.52 microseconds.The resulting waveform generated by the antenna circuit 68 from beingdriven by the bipolar drive signal is illustrated in FIG. 12. As can beseen from the waveform illustrated in FIG. 12, the bipolar drive circuit66 injects energy into the tuned antenna circuit 68 every one-and-a-halfcycles, thereby optimizing the amount of energy injected into theantenna L1 and improving the efficiency of the beacon.

Receiver/Controller

Referring now to FIGS. 13a-13c, a circuit diagram of thereceiver/controller 100 according to the present invention is shown. Thereceiver/controller 100 is adapted to receive the radio frequency signalreceived by the antenna, detect the presence of a beacon signal,demodulate the signal, and determine the content of the serialinformation being transmitted. If the transmission is determined to be avalid beacon code, the receiver/controller 100 further performs theinstructed function corresponding to the function code in the receivedtransmission.

In general, the receiver/controller 100 comprises a microcomputer 102for performing logical and mathematical calculations, receiver circuitryfor receiving and detecting the presence of a beacon transmission, anon-volatile memory device 120 for storing valid identification codes,and output circuitry for controlling various vehicle features such asdoor and trunk locks. The two inputs 116 and 118 from the receivingantenna are provided to an analog receiver circuit that is implementedin the preferred embodiment with a custom integrated circuit 104. Theanalog bipolar receiver circuit 104 comprises a single conversionsuperheterodyne receiver having an intermediate frequency of 4.274 KHz.The signal received from the tuned antenna coil comprises a voltageproportional to the magnetic field received from the beacon. This signalis amplified by a preamplifier, filtered, and mixed with a voltagecontrolled oscillator ("VCO") frequency signal to create an intermediatefrequency ("IF"). The intermediate frequency signal is furtheramplified, filtered, and limited to CMOS signal levels for processing bythe digital data detector circuit 106.

The digital data detector circuit 106, which is also implemented in thepreferred embodiment with a custom integrated circuit, is adapted todetect the presence of a beacon signal and produce an output signal to"wake up" the microcomputer 102. The digital data detection circuit 106also demodulates the PSK encoded signal from the beacon. Datasynchronization circuitry extracts the clock from the resulting Millerencoded signal and clocks the data into the microcomputer. To minimizeaverage power consumption of the receiver/controller 100, the digitaldata detection circuit 106 also signals the microcomputer 102 to go intoa stand-by mode when the beacon is out of range.

In this regard, the bipolar front end receiver circuit 104, the digitaldata detection circuit 106, and the voltage regulator 112 are the onlycircuits that remain active at all times. In the preferred embodimentthe total quiescent current draw for the receiver/controller 100 is lessthan one milliampere. Thus, vehicle battery power is conserved.

The microcomputer 102 used in the preferred embodiment comprises an 840Series 8-bit microcomputer manufactured by National Semiconductor. Themicrocomputer is programmed to decode the Miller encoded signal receivedfrom the digital data detection circuit 106 and compare the resultingbit patterns with the data previously stored in a non-volatile memory120. Based upon the results of this comparison, the microcomputer 102 isfurther programmed to control the activation of various functions suchas door and trunk locks. A separate algorithm stored in themicrocomputer is provided for programming new identification codes intothe non-volatile memory 120. Significantly, in the preferred embodiment,the EEPROM 120 has the capacity for storing and the microcomputer 102 isprogrammed to accept and check for more than one valid beacon code. Inthis manner, several different beacons can be validated and used inconjunction with a single receiver/controller 100.

In addition, the receiver/controller 100 includes interface circuitry112 including a power regulator and a transient suppressor to isolatethe receiver/controller circuitry from noise on the 12-volt batterylines. The interface circuitry 112 also provides various regulated powersupply voltages.

The various additional inputs to the receiver/controller circuit 100serve the following functions. The Key Switch input is grounded when thekey is in the ignition and serves to inhibit the keyless entry system bygrounding the various output lines from the microcomputer 102 to therelay driver circuits that activate the doors and trunk lock mechanisms.The Doorjamb input is used by the microcomputer 102 in combination withthe Key Switch input to automatically lock all of the vehicle doors whenthe beacon is out of receiving range. In particular, the microcomputer102 is programmed to automatically lock all of the vehicle doors apredetermined period of time after the beacon is out of range and all ofthe vehicle doors have been closed, but only if the key is not in theignition. The Hatch or trunk lid input signal is provided as a feedbacksignal to the microcomputer 102 to prevent repetitive actuation of thetrunk or hatch unlock solenoid. The Program input is used to activatethe receiver/controller programming mode. A new identification code canthen be programmed into the non-volatile memory 120 of thereceiver/controller 100. Specifically, to program a new beacon ID codeinto the controller, the Program input line is grounded and a beacon isbrought within range of the receiving antenna. The ID code from thebeacon is thereupon read by the microcomputer 102 into the EEPROM 120.In this manner, if a beacon is lost, a new beacon with a differentidentification code can be provided and the new beacon ID codeconveniently programmed into the receiver/controller 100 by anauthorized service personnel. The Manual Lock and Manual Unlock inputsare provided to the receiver/controller 100 to manually override thesystem regardless of the state of the microcomputer 102. In particular,actuation of the manual lock and/or unlock buttons on the vehicle willoverride the microcomputer 102 and activate the appropriate lock and/orunlock solenoids regardless of the state of the control outputs from themicrocomputer 102.

The output terminals from the microcomputer 102 are provided to variousrelay driver circuits that serve to activate the various lock and unlockdoor and trunk mechanisms. In addition, it is preferred that the relaydriver circuits that interface with external relays or solenoids includeshort circuit protection circuitry to protect the microcomputer 102 andthe relay driver circuits in the event of a short in the external relaysor solenoids.

The various relay driver circuits function in essentially the samemanner. Therefore, the following description of the Unlock Driver's Doorrelay driver circuit can be considered applicable to the remaining relaydriver circuits as well.

When the microcomputer 102 determines from decoding the function codefrom a valid beacon transmission that the driver's door is to beunlocked, the microcomputer 102 produces a logic HI signal at outputport L5 on pin 16. The HI signal on line 124 serves to bias theDarlington transistor Q4 into full conduction, thereby energizing therelay coil of relay RX1. Energization of relay coil RX1 in turn servesto energize a motor (not shown) that is operatively connected to thelock mechanism of the driver's door and is effective to unlock the door.As previously noted, if the manual lock or unlock buttons in the vehicleare actuated, the receiver/controller 100 is overridden regardless ofthe state of the microcomputer 102. In particular, if the manual unlockbutton is actuated, a positive signal pulse is produced on line 126which results in a corresponding positive signal pulse being provided atnode 128, designated "Point A" in the circuit diagram. The positivesignal pulse at node 128 is effective to immediately turn on Darlingtontransistor Q4 as well as Darlington transistor Q5 to thereby energizeboth relays RX1 and RX2 and unlock the driver's side and passengerdoors. Similarly, if the manual lock button is actuated, a positivesignal pulse is provided on line 130 which results in a positive signalpulse being provided at node 132, designated "Point B" in the circuitdiagram. The positive signal pulse at node 132 is effective to turn onDarlington transistor Q6 which in turn energizes relay coil RX3 andlocks all of the vehicle doors.

As also previously noted, the receiver/controller 100 in the presentkeyless entry system is adapted to inhibit system operation whenever theignition key is in the ignition. Specifically, upon insertion of the keyin the ignition, a LO signal pulse is produced on line 134, designated"Point C" in the circuit diagram, which results in a corresponding LOsignal being provided on line 136 to the positive input of a comparator138. This in turn causes the output of comparator 138 to go LO, therebypulling nodes 140 and 142 to ground potential and inhibiting the outputports L5-L7 and G0 of the microcomputer 102. Accordingly, it will beappreciated that when the key is in the ignition, the microcomputer 102is inhibited from activating switching control transistors Q4-Q6 and Q9in the various relay driver circuits, thereby effectively inhibitingoperation of the system.

The additional circuitry 144 shown in the "unlock hatch" drive circuitis provided to detect a short circuit in the remotely located unlockhatch solenoid (not shown) and, in such event, pulse width modulate theDarlington transistor Q9. Optionally, additional circuitry 114 may alsobe provided at the antenna input 118 to provide a continuity test forthe antenna connections.

Turning now to FIG. 14, blocked diagrams of the custom integratedcircuits 104 and 106 utilized to process and demodulate the incomingsignal from the antenna are shown. The analog receiver IC 104 comprisesa very low power amplifier, filter, and converter circuit. The circuitaccepts a narrowband PSK signal at 98.304 KHz from the tuned antennacircuit. The signal is amplified and mixed down to an intermediatefrequency ("IF") of 4.274 KHz in the preferred embodiment. The resultingIF signal is filtered and amplified with the last stage performing alimiting function. The resulting output signal is a 0-5 volt square wavesignal with an approximately 50 percent duty cycle.

With particular reference to the drawing, the two output lines, 116 and118, from the antenna are provided to a differential amplifier 150 whichminimizes common mode noise and amplifies the antenna signal. The outputfrom the differential amplifier 150 is provided to a high Q, very narrowbandwidth, bandpass filter having a center frequency of 98.304 KHz, thefrequency of the beacon transmission. The output from the bandpassfilter 152 is in turn provided to a mixer 154 which converts theincoming signal down to an intermediate frequency signal of 4.274 KHz inthe preferred embodiment. More particularly, the mixer 154 is adapted totake the difference in frequency between the incoming signal from thebandpass filter 152 and the output signal from the voltage controlledoscillator (VCO) circuit 160. The voltage controlled oscillator 160 inthe preferred embodiment is designed to lock onto a frequency signal of102.578 KHz, thus providing the 4.742 KHz differential intermediatefrequency between the VCO frequency signal and the beacon frequencysignal. The IF signal on line 155 is amplified by an amplifier circuit156 and thereafter provided to a limiter circuit 158 which converts thesignal to a square wave signal having approximately a 50 percent dutycycle. The resulting output signal from the limiter circuit 158 on line159 is provided to the digital data detection circuit 106.

The digital data detector 106 performs the following functions. A clockgenerator circuit 162, shown in greater detail in FIG. 15, has anoscillator circuit 184 which uses the 32.768 KHz crystal 110 (FIG. 13a)to generate a 32.768 KHz crystal clock signal (XTAL CLK). The clockgenerator circuit 162 also receives the VCO clock signal, and containsdivider circuits 163 and 165 which divide down the VCO clock signal.These signals are used in the other blocks as timing signals. Thecarrier synchronizer 164 combines with the voltage controlled oscillator160 in the bipolar front end chip 104 to make up a phase lock loopcircuit (PLL). The PLL circuit recreates the carrier frequency necessaryto decode the PSK signal. Rapid frequency acquisition and phasesynchronization with a detected beacon signal are enhanced by afrequency sweep circuit that is controlled by a counter circuit whichcompares the VCO frequency to the crystal oscillator frequency when abeacon is not in range. In this manner, the frequency of the VCO circuitis made to gradually sweep up and down around the 102.578 KHz frequencyto thereby keep the frequency of the VCO signal within the vicinity ofthe expected frequency of a beacon transmission. The lock detectorcircuit 166 is adapted to produce a "μP wake-up signal" when a beaconsignal has been detected and the receiver is properly synchronized tothe beacon signal. In other words, the lock detector circuit 166 isadapted to activate the microcomputer 102 when it is determined that abeacon is present and the beacon signal has been "locked" onto. Themicrocomputer 102 is programmed thereafter to determine if the receivedbeacon signal contains a valid beacon code. The lock detector circuit166 essentially comprises a quadrature lock detector and a digitalfilter. When the phase lock loop circuit acquires both phase andfrequency, the lock detector output from the lock detector circuit 166goes HI. The μP wake-up signal similarly comprises a pulse which causesthe microcomputer 102 to enter its active state. The data extractioncircuit serves to demodulate the received signal to recover the data bitstream. A matched filter is included to remove noise from the signal andadditional circuitry decodes the Miller encoded recovered transition bitstream. The decoding circuitry includes a clock synchronizer toregenerate and synchronize to the data sample clock which is required todemodulate the Miller encoded signal.

Referring now to FIG. 16, a more detailed block diagram of the carriersynchronizer circuit 164 is shown. As previously noted, the purpose ofthe carrier synchronizer 164 is to recreate the carrier frequency signalin proper phase with the received beacon transmission. The regeneratedcarrier signal is required to demodulate the PSK encoded beacontransmission. The carrier synchronizer 164 includes the circuitcomponents which, together with the VCO circuit 160 in the bipolar frontend IC 104, comprise a long loop phase locked loop circuit withfrequency doubling. In particular, the output signal from the bipolarfront end IC 104 on line 159 is provided to a shift register 170 whichshifts the phase of the incoming signal by approximately 90 degrees. Theincoming signal on line 159 together with the phase shifted signal online 171 are then provided to a frequency doubler 172 which mixes thetwo signals to provide a resulting output signal on line 174 having afrequency double that of the input frequency signal on line 159. Thedoubled frequency output signal (DBL OUT) on line 174 is provided to oneinput of a phase error circuit 176 which has its other input connectedto the output of a 90-degree delay circuit 173. The 90-degree delaycircuit 173 delays the VCO DIV 12 signal from the clock generatorcircuit 162 by 90 degrees. The phase error block 176 in the preferredembodiment comprises an exclusive-OR gate which, in a digital system,provides a very close approximation to the mixer used in analog phaselock loop circuits. The duty cycle of the output signal from the phaseerror circuit 176 reflects the amount of phase error between the VCO DIV12 signal and the doubled IF signal on line 174. Exactly 50 percent dutycycle corresponds to a zero phase error. Any other duty cycle causes thecharge pump circuit 178 to produce a voltage signal on output line 180which is fed back to the VCO circuit 160 to correct the VCO frequencyand reduce the phase error. When the phase lock loop circuitry hasacquired both the frequency and phase of the beacon signal, the LOCKDETECT signal provided to the charge pump circuit 178 to disable thefrequency sweep function.

In order to minimize the amount of time it takes for thereceiver/controller to lock onto the beacon transmission, a 32.768 KHzwatch crystal 110 (FIG. 13a) comparable to the watch crystal 46 in thebeacon, is included in the receiver circuitry to provide a fixedfrequency signal that closely approximates the expected frequency of thebeacon transmission. Returning to FIG. 15, the clock generator circuit162 includes a 32.768 KHz oscillator circuit 184 that is connected tothe watch crystal 110 and is adapted to produce a fixed frequency outputsignal on line 185 (XTAL CLK) substantially equal to the crystalfrequency of the beacon signal. The XTAL CLK signal on line 185 isprovided through a pulse generator 186 which divides the frequency ofthe XTAL clock signal by a factor of 3. The resulting pulse generatoroutput signal on line 182 is provided to the carrier synchronizercircuit 164 and is used to keep the frequency of the output signal fromthe voltage controlled oscillator 160 within the vicinity of 102.578KHz. In particular, the pulse generator output signal on line 182,together with the VCO DIV 3 signal, are provided to a VCO frequencycounter 178 and VCO frequency counter decoder 179 which essentiallycompare the frequency of the VCO signal to the frequency of the XTALclock signal. The output is then provided to a sweep control latchcircuit 188 which in turn directs the charge pump circuit 178 to slowlysweep the frequency of the output signal from the voltage controlledoscillator 160 up and down within a couple hundred Hertz of 102.578 KHz.In other words, the VCO frequency counter, decoder, and sweep controllatch circuits 178-180 serve to provide a feedback loop for the chargepump circuit 178 when no beacon is within range of the receiver antennato thereby slowly dither the frequency of the VCO signal within thevicinity of 102.578 KHz. In this manner, the receiver/controller 100 isable to rapidly lock onto the phase of the beacon frequency signal whena valid beacon signal is detected. Once a beacon comes within range ofthe receiving antenna, the charge pump circuit 178 is primarilyinfluenced by the phase error circuit 176 and thus functions as aconventional phase lock loop circuit. Accordingly, circuits 178-180 haveno effect on the operation of the phase lock loop circuitry once abeacon is within range. This is accomplished by the lock detectorcircuit 166 which produces a LOCK DETECT output signal on line 190 whena beacon signal is detected that is provided to the charge pump circuit178 and is effective to disable the frequency sweep function.

Referring now to FIG. 17, a detailed block diagram of the lock detectorcircuit 166 is shown. As previously noted, the lock detector circuit 166determines when the receiver has detected and properly synchronized to abeacon signal and produces a μP wake-up signal to turn on themicrocomputer 102. The microcomputer 102 thereupon determines if thereceived signal is from a valid beacon. The lock detector circuit 166comprises an exclusive-OR quadrature circuit 192, a lock detectorinterval counter 194, a lock detector low pass filter and comparator196, a hold-on circuit 198, and a μP wake-up circuit 200. In particular,the DBL OUT signal on line 174 from the carrier synchronizer circuit 164and the VCO DIV 12 from the clock generator circuit 162 are provided tothe inputs of the exclusive-OR quadrature circuit 192. It will berecalled that the DBL OUT signal on line 174, which has a frequencytwice the frequency of the IF signal on line 159, has been phase shifted90 degrees relative to the IF signal. Thus, the VCO DIV 12 signal is 90degrees out of phase with the DBL OUT signal on line 174, thereby makingthe circuit a true quadrature detector. When a beacon signal is presentand the PLL circuit has acquired both proper frequency and phase, theexclusive-OR quadrature circuit 192 will provide a HI output signal online 193 most of the time. When the PLL circuit is unlocked, the outputsignal on line 193 will toggle up and down with an average HI time ofapproximately 50 percent. Lock detector circuits 194 and 196 are adaptedto sample the quadrature output line 193 over a fixed period of time andcount the number of samples which are in quadrature. In the preferredembodiment, lock is indicated when 384 or more samples are in quadratureout of a possible 511. Specifically, the lock detector counter 196 andinterval counter 194 are reset at the same time and clocked by the sameVCO DIV 6 clock signal. Interval counter 194 is incremented each clockpulse while the lock detector counter 196 is incremented at a clockpulse only if the output signal on line 193 is HI. If the count total inlock detector counter 196 is equal to or greater than 384 at the time ofthe next reset pulse, the hold-on circuit 198 is activated. Hold-oncircuit 198 comprises a retriggerable one-shot circuit that is adaptedto maintain the lock condition to the microcomputer 102 forapproximately 500 milliseconds after it is no longer indicated by thecounters 194 and 196. In this manner, the lock detector circuit 166ignores dropouts in the beacon signal or short-term noise bursts. The μPwake-up circuit 200 comprises a D flip-flop that produces a 7.8millisecond output pulse on line 202 that is provided to themicrocomputer 102 in response to the production of the LOCK DETECToutput signal from the hold-on circuit 198.

Turning now to FIG. 18, a detailed block diagram of the data extractioncircuit 168 is shown. As previously noted, the data extraction circuit168 demodulates the incoming PSK encoded signal and runs the resultingdata bit stream through a matched filter to remove noise. Initialdecoding of the Miller encoded data is performed and the synchronizingdata clock is generated to decode the Miller code and clock the data bitstream into the microcomputer 102. Specifically, the data extractioncircuitry 168 includes a data extraction circuit 204 which comprises anexclusive-OR gate having the PSK encoded signal on line 159 provided toone input and the synchronized VCO DIV 24 signal provided to its otherinput. Once locked onto the beacon signal, it will be appreciated thatthe frequency of the VCO signal (102.578 KHz) divided by 24 equals thesame 4.274 KHz signal as the intermediate frequency (IF) signal on line159. It will further be appreciated, therefore, that the synchronizedVCO DIV 24 clock signal contains the recreated carrier phase locked tothe intermediate frequency signal on line 159. The data extractionexclusive-OR gate 204 demodulates the PSK encoded signal and the outputis filtered through a digital matched filter circuit 206. The early-lategate data synchronizer circuit 208 generates the clock signal used todecode the Miller encoded data and clock the data bit stream into themicrocomputer 102. Initial decoding of the Miller encoded transition bitstream is then performed by the transition data bit stream generator210. The serial data bit stream is provided to the microcomputer 102 onoutput line 214. The μ P data handshake circuit 212 interfaces with themicrocomputer to control the serial inputting of the data bit streaminto the microcomputer 102. In the preferred embodiment, final decodingof the data bit stream is performed in the microcomputer 102 bysoftware.

While the above description constitutes the preferred embodiment of theinvention, it will be appreciated that the invention is susceptible tomodification, variation, and change without departing from the properscope or fair meaning of the accompanying claims.

What is claimed is:
 1. A keyless entry system for a vehicle comprising:aportable beacon adapted to be carried by the operator of the vehicle andincluding transmitter means for transmitting at a predeterminedfrequency an encoded sinusoidal beacon signal comprising a plurality ofdata bits identifying said beacon, said beacon signal including asinusoidal waveform having associated waveform peaks, said beaconfurther including an antenna drive circuit and a tuned antenna circuitadapted to resonate at said predetermined frequency, said tuned antennacircuit generating the sinusoidal waveform when excited by a drivesignal from said antenna drive circuit, said antenna drive circuitproviding a pulsed driving signal comprising periodic driving signalpulses interspersed with off periods to said tuned antenna circuit, saiddriving signal pulses being produced at a predetermined periodic raterelated to said predetermined frequency so that the driving signalpulses substantially coincide with the peaks in the beacon signalwaveform, and further wherein the duration of the driving signal pulsesare substantially less than the off periods between successive drivingsignal pulses; and receiver means associated with said vehicle andincluding antenna means for receiving said beacon signal, signalprocessing means for decoding said beacon signal to recover saidplurality of data bits, and controller means for determining from saidplurality of recovered data bits whether said beacon signal correspondsto a valid beacon in activating a predetermined function associated withsaid vehicle in response thereto.
 2. The keyless entry system of claim 1wherein said signal processing means includes means for creating asignal having a frequency related to said carrier frequency and properlyphased with respect to said beacon signal comprising phase lock loopcircuit means for locking onto the frequency and phase of said beaconsignal when a beacon is within range of said antenna means and sweepcircuit means causing the frequency of said phase lock loop circuitmeans to gradually sweep up and down around a frequency related to saidpredetermined carried frequency when a beacon is not within range ofsaid antenna means.
 3. The keyless entry system of claim 1 wherein saidfrequency signal is a phase encoded radio frequency signal beingtransmitted at a single predetermined carrier frequency with the phaseof the beacon signal shifted to distinguish between a logical "1" and alogical "0".
 4. The keyless entry system of claim 3 wherein said antennameans is tuned to said predetermined carrier frequency to magneticallycouple the energy of said beacon signal into said receiver means.
 5. Thekeyless entry system of claim 1 wherein said beacon further includes aportable power source, motion sensing means for sensing physical motionof said beacon, and circuit means responsive to said motion sensingmeans for activating said transmitter means to transmit said beaconsignal.
 6. The keyless entry system of claim 1 wherein saidpredetermined rate is approximately every 1.5 cycles of the frequencybeacon signal.
 7. The keyless entry system of claim 5 wherein thecircuitry in said beacon has a current drain on said portable powersource in the microamp range when said beacon is stationary.
 8. Thekeyless entry system of claim 1 wherein said beacon signal includesfunction information identifying the predetermined function to beperformed by said receiver means.
 9. The keyless entry system of claim 8wherein said beacon signal comprises a digital code having apredetermined number of identification and function bits.
 10. Thekeyless entry system of claim 9 wherein said beacon signal furtherincludes an error correction code comprised of a number of bits equal tosaid predetermined number of identification and function bits.
 11. Thekeyless entry system of claim 10 wherein said beacon signal is Millerencoded.